Plasma source for etching

ABSTRACT

An apparatus for generating plasma is disclosed. The apparatus comprises: a plasma chamber; pairs of parallel plate electrodes; and a power supply for applying high-frequency powers on the pairs of electrodes. The frequencies of the high-frequency powers and the phase difference between the high-frequency powers are adjusted so as to cause each of electrons in the plasma to move in a circular path. A dense and highly uniform plasma is generated at a low pressure level, by utilizing the phenomenon of the oscillation, revolution or cycloidal motion of electrons in a high-frequency electric field formed between the parallel plate electrodes. This plasma is suitable for etching in the LSI fabrication process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma technology, and moreparticularly, to a plasma source for processing in the field ofsemiconductor fabrication technology.

2. Description of the Prior Art

The progress in the large scale integrated circuit (LSI) technology isbringing about a change comparable to that of the industrial revolution.A high packing density of the LSI has been realized through a reductionin the device dimensions, an improvement in device structures andenlargement of chip surface areas. In recent years, the devicedimensions has been reduced to the light wavelength region, and the useof excimer laser or soft x-rays in lithography is being studied.

In the field of semiconductor fabrication processing, the plasmatechnology is widely used for dry etching, chemical vapor deposition,sputtering and so on. Especially, dry etching technique plays animportant role in the formation of fine patterns, as well as lithographytechnique.

Dry etching is a process for removing unnecessary portions of the solidsurface (e.g., a surface of a semiconductor substrate, layers depositedon a substrate, and so on) by utilizing chemical or physical reactionsat the interface between plasma and the solid surface. The plasma-solidsurface reactions are induced by the interaction between the solidsurface and free radicals, ions, etc., generated in the plasma.

Reactive ion etching (RIE), which is the most widely used as a dryetching technique, removes the unnecessary portions of the surface ofthe sample by an etching reaction that occurs when the sample is exposedto the high-frequency discharge plasma of a suitable gas. The necessaryportions are normally protected by a photoresist pattern used as a mask.In order to improve the fineness of the pattern, it is necessary toalign the directionality of the ions. This cannot be achieved withoutreducing the ion scattering in the plasma. In making the directionalityof the ions uniform, decreasing the pressure of plasma is effective inincreasing the mean free path of the ions. However, it is difficult fora low pressure gas to be discharged by a high-frequency power (A.C.power). To solve this problem, magnetron reactive ion etching andelectron cyclotron resonance (ECR) etching techniques, in which amagnetic field is applied on the plasma chamber, have been developed togenerate a low pressure gas discharge.

FIG. 14 shows a typical prior art magnetron discharge etcher forreactive ion etching (RIE). In this etcher, a gas controller 82 passesinto a metal chamber 81 and introduces the reactive gas, while thepressure is appropriately controlled by an exhaust system 83. An anode84 is provided at the top of the chamber 81. A sample stage 85 (whichserves as a cathode) is provided at the bottom. An RF power source 87 isconnected to the sample stage 85 via an impedance-matching circuit 86 tofacilitate a high-frequency discharge between the sample stage 85 andthe anode 84. A rotating magnetic field is applied in the chamber 81 bya pair of opposing AC electromagnets 88 mounted on the sides and whosephases are shifted 90 degrees from each other, thus facilitating adischarge at a low pressure. The cycloid motion of the electron inducedby the magnetic field improves ionization efficiency.

Although the purpose of using the above magnetron discharge and ECRdischarge is to increase the plasma density, the discharges are stillnot sufficiently capable of generating uniformly a highly dense plasmaover the entire surface of the sample. In the prior art magnetronreactive ion etching etcher, local plasma densities are regarded asuniform by means of a rotating magnetic field when averaging them overlong time. In fact, however, the instantaneous plasma densities are notuniform. Therefore, when a wafer is exposed to the plasma for thefabrication of MOSLSI in the prior art etcher, local potentialdifferences of the plasma may cause the gate oxide film of the MOSLSI tobreak down. In the ECR etcher, a magnetic field is distributed radiallyin the chamber, so that the local deviation in the plasma density occursand it causes a non-uniform etching and local potential differences.

Due to the non-uniformity of the plasma generated in the prior artetcher, it is difficult to achieve a high-yield production of MOSLSI.Further, it is difficult to achieve a high repeatability in the etchingof ultrafine-pattern LSI devices having thin gate oxide films and theetching of a large sample (e.g., a large-diameter wafer).

To generate a high-density plasma, and to lower the self-bias in theplasma for reducing damage to the sample caused by high energy ions, ahigh-frequency power (ranging from 100 to 200 MHz) may be superimposedwith a 13.56 MHz power on the parallel plate electrode of the magnetronetcher. Even in this method, it is difficult to improve the uniformityof the plasma. Therefore, it is not sufficient to solve the problemscaused by The non-uniform plasma.

SUMMARY OF THE INVENTION

The apparatus for generating plasma of this invention, which overcomesthe above-discussed and numerous other disadvantages and deficiencies ofthe prior art, comprises: a plasma chamber; a pair of electrodes forgenerating plasma in said plasma chamber; and power supply means forapplying a high-frequency power on said pair of electrodes, the periodof the high-frequency power being shorter than the time required forelectrons contained in said plasma To travel between said pair ofelectrodes.

In a preferred embodiment, said high-frequency power has the frequencyof 50 MHz or more.

According to the invention, an apparatus for generating plasma isprovided, said apparatus comprising: a plasma chamber; at least threeelectrodes for generating plasma in said chamber, each one of saidelectrodes facing toward the others; and power supply means for applyinghigh-frequency powers on at least two pairs of said electrodes, thefrequency of each of said high-frequency powers being an integralmultiple of the lowest frequency among said high-frequency powers, thephases of said high-frequency powers having a fixed relation with eachother.

According to the invention, an apparatus for generating plasmasprovided, said apparatus comprising: a plasma-chamber; a first pair ofparallel plate electrodes: a second pair of parallel plate electrodes,said second pair of parallel plate electrodes being substantiallyperpendicular with said first pair of parallel plate electrodes; andpower supply means for applying high-frequency powers on said first andsecond pairs of said electrodes, the frequency of each of saidhigh-frequency powers being an integral multiple of the lowest frequencyamong said high-frequency powers, the phase difference between saidhigh-frequency powers applied on said first and second pairs of saidelectrodes being 90 degrees.

According to the invention, an apparatus for generating plasma isprovided, said apparatus comprising: a plasma chamber; a first pair ofparallel plate electrodes; a second pair of parallel plate electrodes,said second pair of parallel plate electrodes being substantiallyperpendicular with said first pair of parallel plate electrodes; a thirdpair of parallel plate electrodes, said third pair of parallel plateelectrodes being substantially perpendicular with said first pair ofparallel plate electrodes and said second pair of parallel plateelectrodes; and power supply means for applying high-frequency powers onsaid first, second and third pairs of said electrodes, the frequency ofeach of said high-frequency powers being an integral multiple of thelowest frequency among said high-frequency powers, the phase differencesbetween said high-frequency powers applied on said first, second andthird pairs of said electrodes being 120 degrees.

In a preferred embodiment, said apparatus further comprises magneticfield applying means for forming a magnetic field to confine the plasma.

In a preferred embodiment, said power supply means comprises: a singlesignal source for producing a high-frequency signal; at least twoamplifiers for receiving said high-frequency signal from said singlesignal source and amplifying said high-frequency signal to saidhigh-frequency powers; and a phase locking controller for fixing thephase difference between said high-frequency powers.

According to the invention, a plasma etcher is provided, said plasmaetcher comprising: a chamber; a pair of electrodes for generating plasmain said chamber; a holder for holding at least one wafer, said holderdisposed in said chamber for exposing the surface of said wafer to saidplasma; and power supply means for applying a high-frequency power onsaid pair of electrodes, the period of the high-frequency power beingshorter than the time required for electrons contained in said plasma totravel between said pair of electrodes.

In a preferred embodiment, said high-frequency power has the frequencyof 50 MHz or more.

According to the invention, a plasma etcher is provided, said plasmaetcher comprising: a chamber; at least three electrodes for generatingplasma in said chamber, each one of said electrodes facing toward theothers; a holder for holding at least one wafer, said holder disposed insaid chamber for exposing the surface of said wafer to said plasma; andpower supply means for applying high-frequency powers on at least twopairs of said electrodes, the frequency of each of said high-frequencypowers being an integral multiple of the lowest frequency among saidhigh-frequency powers, the phases of said high-frequency powers having afixed relation with each other.

According to the invention, a plasma etcher is provided, said plasmaetcher comprising: a chamber; a first pair of parallel plate electrodes:a second pair of parallel plate electrodes, said second pair of parallelplate electrodes being substantially perpendicular with said first pairof parallel plate electrodes; power supply means for applyinghigh-frequency powers on said first and second pairs of said electrodes,a holder for holding at least one wafer, said holder disposed in saidchamber for exposing the surface of said wafer to plasma; and thefrequency of each of said high-frequency powers being an integralmultiple of the lowest frequency among said high-frequency powers, thephase difference between said high-frequency powers applied on saidfirst and second pairs of said electrodes being 90 degrees.

According to the invention, a plasma etcher is provided, said plasmaetcher comprising: a chamber; a first pair of parallel plate electrodes;a second pair of parallel plate electrodes, said second pair of parallelplate electrodes being substantially perpendicular with said first pairof parallel plate electrodes; a third pair of parallel plate electrodes,said third pair of parallel plate electrodes being substantiallyperpendicular with said first pair of parallel plate electrodes and saidsecond pair of parallel plate electrodes; and power supply means forapplying high-frequency powers on said first, second and third pairs ofparallel plate electrodes, the frequency of each of said high-frequencypowers being an integral multiple of the lowest frequency among saidhigh-frequency powers, the phase differences between said high-frequencypowers applied on said first, second and third pairs of parallel plateelectrodes being 120 degrees, one of said parallel plate electrodesfunctioning as a holder for holding at least one wafer for exposing saidwafer to plasma.

In a preferred embodiment, said plasma etcher further comprises magneticfield applying means for forming a magnetic field to confine the plasma.

In a preferred embodiment, said power supply means comprises: a singlesignal source for producing a high-frequency signal; at least twoamplifiers for receiving said high-frequency signal from said singlesignal source and amplifying said high-frequency signal to saidhigh-frequency powers; and a phase locking controller for fixing thephase difference between said high-frequency powers.

In a preferred embodiment, the temperature of said holder is the lowestin said chamber.

In a preferred embodiment, an electronegative gas is introduced intosaid chamber, and is discharged by said high-frequency powers forgenerating the plasma.

In a preferred embodiment, said electronegative gas includes a gasselected from the group of SF₆, Br₂, Cl₂, I₂ and O₂.

In a preferred embodiment, said plasma etcher further comprises biassupplying means for applying a bias on said holder.

In a preferred embodiment, said bias supplying means includes means forapplying an AC power on said holder for generating a DC bias.

In a preferred embodiment, said bias supplying means starts to apply abias on said holder after the plasma is generated.

In a preferred embodiment, said plasma etcher further comprises atemperature controller for controlling the temperature of said holder.

In a preferred embodiment, said plasma etcher further comprises: atleast one magnetic field strength detected for detecting the strength ofmagnetic field formed by motions of electrons of the plasma; and meansfor controlling said high-frequency powers according to said strength ofthe magnetic field.

According to the invention, a method of increasing the uniformity ofplasma is provided, in which electrons in the plasma are caused to movein circular paths by applying an electric field on said electrons, saidelectric field being formed by combining at least two high-frequencyelectric fields, the phase difference of said at least twohigh-frequency electric fields being fixed at constant.

According to the invention, a method for plasma etching is provided, inwhich electrons in plasma are caused to move in circular paths byapplying an electric field on said electrons, said electric field beingformed by combining at least two high-frequency electric fields, thephase difference of said at least two high-frequency electric fieldsbeing fixed at constant, and a sample to be etched is exposed to saidplasma for being etched.

Thus, the invention described herein makes possible the objectives of(1) providing an apparatus for generating a dense and highly uniformplasma at a low pressure; (2) providing a plasma etcher for achievingsuperior fine patterning and good uniformity at a low pressure withoutbreakdown of thin insulators of etched devices; (3) providing a methodof increasing the uniformity of plasma; and (4) providing a method forplasma etching in which a sample is etched uniformly without beingdamaged.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIG. 1 is a diagram showing the configuration of a plasma etcher of afirst embodiment according to the invention;

FIGS. 2A and 2B are diagrams for explaining the path of electron in thechamber of the plasma etcher of the first embodiment;

FIG. 3 is a graph showing the dependence of the distance electronstravel in one period on frequency;

FIGS. 4A through 4E are diagrams showing the relationship between phasedifference (Φ) and Lissajous waveforms;

FIGS. 5A and 5B are diagrams showing the magnetic flux distribution andthe electron motions in a prior ark magnetron etcher;

FIG. 6 is a diagram showing the electron motions in a plasma etcher ofthe invention;

FIG. 7A is a cross section of a sample to be etched, and FIGS. 7B and 7Care graphs for showing a magnetic field strength distribution and anetch rate distribution in a prior art magnetron etcher;

FIG. 8A is a cross section of a sample to be etched, and FIG. 8B is agraph for showing an etch rate distribution in a plasma etcher of theinvention;

FIG. 9 is a table showing a comparison of the plasma etcher of theinvention with the prior art plasma etcher;

FIG. 10 is a diagram showing the configuration of a plasma etcher of asecond embodiment according to the invention;

FIG. 11 is a diagram for explaining the path of electrons in the chamberof the plasma etcher of the second embodiment;

FIG. 12 is a diagram showing the configuration of a plasma etcher of athird embodiment according to the invention;

FIG. 13 is a diagram showing the configuration of a plasma etcher of afourth embodiment according to the invention; and

FIG. 14 is a diagram showing an RIE etcher employing a prior artmagnetron discharge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a plasma etcher of a first embodiment according to theinvention. The plasma etcher comprises: an etching chamber (plasmachamber) 1; a first pair of parallel plate electrodes 5 and 6; and asecond pair of parallel plate electrodes 7 and 8. The two pairs ofparallel plate electrodes 5, 6, 7 and 8 are arranged in the chamber 1 sothat the second pair of parallel plate electrodes 7 and 8 aresubstantially perpendicular with the first pair of parallel plateelectrodes 5 and 6, as shown in FIG. 1. On each of the two pairs of theparallel plate electrodes 5, 6, 7 and 8, a 300 MHz high-frequency powerisapplied. The phase of the 300 MHz high-frequency power applied betweenthe parallel plate electrodes 5 and 6 differs by approximately 90degrees fromthe phase of the power applied between the parallel plateelectrodes 7 and 8. The plasma etcher further comprises a sample stage(holder) 2 serving as a parallel plate electrode on which a 50 MHzhigh-frequency power is applied, and an opposing electrode 4. The samplestage 2 supports a sample(a semiconductor wafer) to be etched. In thespace surrounded by the samplestage 2 and the parallel plate electrodes4 to 8, the plasma is generated. The etching gas is introduced into thechamber 1 through a mass flow controller (not shown) and an inlet (notshown) of The chamber 1. The internal pressure of the chamber ismaintained at 0.1 to 10 Pa by a turbo pump (not shown).

Amplifiers 9 and 10 supply high-frequency power to the parallel plateelectrodes 5, 6, 7 and 8 via matching circuits 12 and 13. The amplifiers9and are controlled by a phase-locking mechanism 11 so that there is afixedphase difference (90 degrees) between the high-frequency powers. Inorder to make the frequencies of the high-frequency powers equal, asignal generated by a single signal source 70 is used. The signal isamplified tothe high-frequency powers having the same frequency anddifferent phases inthe amplifiers 9 and 10. Further, a 50 MHzhigh-frequency power is amplified by an amplifier 14 and supplied to thesample stage 2 through a matching circuit 15.

As explained below, the force exerted on an electron in the plasma bythe electric field formed between the two pairs of parallel plateelectrodes 5to 8 causes the electron to move in a circular path. Thecircular motions of many electrons generate a magnetic field which isdetectable by using amagnetic field detector such as a Hall element. Thestrength of the magnetic field is proportional to the plasma density(the electron density). Therefore, in order to control the plasmadensity, the high-frequency power applied on the parallel plateelectrodes 5 to 8 can be adjusted according to the magnetic fieldstrength measured by a Hall element 16 which is attached to the plasmaetcher.

Referring now to FIGS. 2A and 2B, electron motions in the plasma etcherof the invention is explained below. FIG. 2A shows the typical path ofan electron e when a high-frequency power is applied on the pair ofparallel plate electrodes 5 and 6. The electron e moves in the directionof its initial electron velocity (which depends on the magnitude of thekinetic momentum of the electron) while being oscillated by thehigh-frequency electric field between the electrodes 5 and 6.

FIG. 3 shows the distance the electron having an energy of 20 eV travelsduring one period of the high frequency power, as a function of thefrequency. For example, an electron traveling in the X direction with anenergy of 20 eV moves approximately 6 cm in 20 nanoseconds (one periodof the 50 MHz high-frequency power). Assuming the distance betweenparallel plate electrodes 5 and 6 is 30 cm, then the electron oscillatesapproximately five times while traveling the distance. Since an electronhaving a larger energy travels at a higher speed, the number of theoscillations is decreased.

To ionize a gas, the electron energy of about 15 eV or greater isgenerallyrequired, although it depends on the kinds of the gas.Ionization of the gas occurs as a result of collisions between electronsand gas molecules (neutrals). Since the probability for the collisionincreases as the free path of the electrons in a unit time increases,ionization efficiency is increased as the free path of the electronsincreases. According to the invention, the free path of the electrons ina unit time is increased by causing the electrons to oscillate or tomove in a circular path by means of a high-frequency electric fieldwithout applying a magnetic field, thereby improving ionizationefficiency. It can also be thought of as effectively increasing thecollision cross section with the gas molecules by the oscillation orrevolution of the electrons.

Since the distance between the parallel plate electrodes 5 and 6 isnormally several tens of centimeters, a high frequency greater thanapproximately 50 MHz is required to improve ionization efficiency.However, even if the free paths of electrons are not increased by afrequency lower than 50 MHz, the probability of their colliding with thechamber wall and being annihilated decreases with the oscillation orrevolution of lower energy electron. Therefore, a decrease in electrondensity is prevented and ionization efficiency remains high.

A high-frequency power of 50 MHz or greater is used only for a microwavedischarge in prior art plasma etchers. The frequency of a microwavepower supplied from a magnetron is in the gigahertz band.

FIG. 2B shows electron motion in the electric field formed when thehigh-frequency powers of the same frequency and whose phases differ by90 degrees are applied on the two pairs of parallel plate electrodes 5to 8. The path of the electron e, as shown in FIG. 2B, is the same asthe so-called Lissajous waveform seen when signals of the same frequencywhosephases are shifted 90 degrees are input to the X signal input end Ysignal input of an oscilloscope. The Lissajous waveform becomes adifferent wave due to the phase difference (Φ) of the high-frequencypowers input to X and Y. The relationship between the phase difference(Φ) and the Lissajous waveform is shown in FIGS. 4A to 4E. Thus,ionization efficiencyis improved by using the electric field tooscillate or to move the electron in a circular path.

AS shown in FIG. 5A, in a prior art magnetron etcher using a rotatingmagnetic field, the magnetic flux distribution 20 immediately above thesample stage at a certain instant would be non-uniform. Since the radiusof the path of the electrons e (black dots in FIG. 5B) is inverselyproportional to the strength of the magnetic field, the radius of thepathof the electrons e increases in size where the magnetic field isweak, and so the electrons e readily collide with the chamber wall andare annihilated. This reduces the electron density in locations wherethe magnetic field is weak and, in turn, lowers the plasma density. Inthe prior art magnetron etcher, non-uniformities in the plasma densityoccur and result in non-uniform etching and damage to the sample 3 beingetched.

In the plasma etcher of the invention, the electric field is uniform intheentire area between the parallel plate electrodes 5, 6, 7 and 8.Therefore,the radius of the electron path is equal everywhere as shownin FIG. 6, thus making the plasma density equal throughout the entireplasma generation area 21. The reaction products (e.g., etchants)generated from the reactive gas plasma in the plasma generation area 21impinges uniformly on the entire surface of the sample 3 (not shown inFIG. 6). This results in uniform etching of the entire sample facing theplasma generation area 21 end little damage to the sample 3 due to thecharge build-up. Moreover, a high plasma density provides a fast etchingrate.

FIG. 7A shows an etching process in the prior art magnetron etcher usingthe rotating magnetic field. As shown in FIG. 7A, a boro-phosphosilicate glass (BPSG) 31 formed on a silicon substrate 30 is exposed, asa sample to be etched, to the plasma in the magnetron etcher. Portionsof the BPSG 31 are covered with a photoresist pattern 32. According tothe magnetron etcher using the rotating magnetic field, the magneticfield is not uniform. As shown in FIG. 7B, when the magnetic fieldintensity immediately above the silicon substrate 30 is at a minimum inthe center of the sample stage (not shown in FIG. 7B), the flux I of theions (reaction products for etching) which impinge on the center surfaceof thesilicon substrate 30 is sparse as shown in FIG. 7A. This isbecause the flux I of the ions is proportional to the plasma densitydistribution corresponding to the magnetic field intensity distribution.The etching rate of the BPSG 31 nearly follows the ion flux I andbecomes non-uniform,as shown in FIG. 7C. Further, non-uniformities ofthe plasma density also bring about damage due to deviations in a chargedistribution.

As shown in FIG. 8A, in the plasma etcher of the invention, a uniformplasma is generated as described above. Therefore, the flux II of theionswhich impinge on the surface of the silicon substrate 30 is alsouniform. This results in a highly uniform etching rate as shown in FIG.8B. Also, there Is little deviation of the charge build-up, so thatthere is very little damage to devices on the wafer to be etched.

In the etching process using the plasma etcher of the invention, we useda gas containing CHF₃ +O₂, CF₄ +CH₂ F₂, or other Freon gas as a basegas. The gas was introduced into the chamber and the pressure was set at0.1 to 10 Pa. The etching rate was 100 to 350 nm/min.

The method of etching according to the invention is particularly suitedto the formation of submicron patterns and the etching of large-diametersemiconductor wafers 6 or 8 inches across. This is because the pressureinthe chamber can be kept low in the method. That is, there is littleion scattering under the low pressure conditions and the etching ratedoes notdepend on pattern size. This results in little pattern shift(so-called CD loss) from the photoresist pattern in the etching process.It is also because the chamber can be easily made large withoutdecreasing the uniformity of the plasma.

In another experiment using the plasma etcher of the invention, a gaswas used in which a small amount of oxygen was mixed with SF₆, so as toetch a phosphorous-doped polycrystalline silicon.

The high effectiveness of the invention was confirmed from experimentalresults when an electronegative gas such as SF₆, oxygen, chlorineandiodine was used as an etching gas. Since the electron density is lowand the resistance is high in the high-frequency plasma of theelectronegativegas, the potential gradient in the plasma is largecompared to that of an electropositive gas plasma, and so theeffectiveness of the invention is particularly great.

In this case, as well, the electric field formed between the parallelplateelectrodes is uniform. Therefore, a uniform plasma is obtained,which yields uniform etching. Also, since there are almost no localdeviations in the plasma, damage to the device such as breakdown of thegate oxide inMOSLSI is greatly decreased. Moreover, etching rates of 200to 400 nm/min are also obtained.

The strength of the magnetic field generated by the revolution of theelectrons in the plasma is detected by the Hall element 16. Since themagnetic field strength is proportional to the plasma density (electrondensity), the condition of the plasma is maintained constant byincreasingor decreasing the high-frequency power so that the measuredintensity of the magnetic field remains at a set level. By this means, agood plasma condition can be reproduced since control is performed bydirect feedback from the plasma condition.

Although the oxide film and polycrystalline silicon are etched using theplasma etching technique according to the invention, other materials,for example, silicon compounds, metals such as Aluminum alloys, or anindividual resists in multilayer resists may be etched. In these cases,effectiveness of the invention is further improved when suchelectronegative gases as SF₆, Br₂, Cl₂, I₂ and O₂are used.

FIG. 9 shows a comparison of characteristics of a prior art dry etchingwith those of the dry etching of the invention. It can be seen that thedry etching of the invention is far superior to the prior art method.

As described above, according to this embodiment, a high-frequency powerofa first frequency is applied on the first opposing electrodes 5 and 6and ahigh-frequency power of the first frequency with a phase shiftedapproximately 90 degrees from that for the parallel plate electrodes 5and6 is applied on the second opposing electrodes 7 and 8 to cause theelectrons in plasma generation area to move in a circular (includeselliptical) path, whereby a high-density, highly uniform plasma isgenerated in spite of a low pressure, and by controlling the applicationof the 50 MHz high frequency power on the sample stage 2, theselectivity ratio to the substrate could be controlled during etching.Further, since there is almost no local deviations in the plasma, damageto the etched sample can be greatly decreased.

In this embodiment, the phase difference between the high-frequencypowers was fixed at 90 degrees because this provides the greatest powerinput efficiency. However, the invention is still effective at phasedifferencesother than 90 degrees.

FIG. 10 shows a plasma etcher of a second embodiment according to theinvention. The plasma etcher comprises: an etching chamber (plasmachamber) 41; a first pair of parallel plate electrodes 44 and 46; and asecond pair of parallel plate electrodes 45 and 47. The two pairs ofparallel plate electrodes 44, 45, 46 and 47 are arranged in the chamber41so that the second pair of parallel plate electrodes 45 and 47 aresubstantially perpendicular with the first pair of parallel plateelectrodes 44 and 46, as shown in FIG. 10. On each of the parallel plateelectrodes 44 and 45, a 150 MHz high-frequency power and 300 MHzhigh-frequency power are applied respectively. The parallel plateelectrodes 46 and 47 are grounded. The phase of the high-frequency powerapplied on the parallel plate electrode 44 differs by approximately 0degree from the phase of the power applied on the parallel plateelectrode45. The plasma etcher further comprises a sample stage 42serving as an electrode on which a 13.56 MHz high-frequency power isapplied, and an opposing electrode 43 which is grounded. The samplestage 42 supports a sample (a semiconductor wafer to be etched). In thespace surrounded by the sample stage 42 and the parallel plateelectrodes 44 to 47, the plasmais generated.

Amplifiers 51 and 52 supply high-frequency powers to the parallel plateelectrodes 44 and 45, respectively, via matching circuits 49 and 50. Theamplifiers 51 and 52 are controlled by a phase-locking mechanism 54 sothat there is a fixed phase difference (0 degree) between thehigh-frequency power. Although the phases of the powers applied on theparallel plate electrodes 44 and 45 are the same in this embodiment,different phases can be used. A 13.56 MHz high-frequency power isamplified by an amplifier 53 and supplied to the sample stage 42 throughamatching circuit 48. Coils 55 surrounding the chamber 41 form a cuspmagnetic field, thereby confining the plasma within the center potion ofthe chamber 41.

The etching gas is introduced into the chamber 41 through a mass flowcontroller (not shown) and an inlet (not shown). The internal pressureof the chamber is maintained at 0.1 to 10 Pa during etching by a turbopump (not shown).

In order to maintain the frequency ratio between the two high-frequencypowers constant, a signal generated by a single signal source is used.Thesignal is multiplied and amplified to the high-frequency powershaving the different frequencies and phases in the amplifiers 51 and 52.

This embodiment differs from the first embodiment in that thehigh-frequency powers having different frequencies of 150 MHz and 300MHz are each applied on the parallel plate electrodes 44 and 45 and theplasmais closed by the cusp magnetic field generated by the coils 55 inthe second embodiment.

FIG. 11 shows a plane projection of the path of the electrons e An thechamber of the plasma etcher of the second embodiment. As expected, thehigh-frequency powers applied on the parallel plate electrodes 44 and 45cause each of the electrons to move in a circular path (precisely, theshape of a figure eight). According to the embodiment, a high ionizationefficiency and a high plasma density are obtained in spite of a lowpressure, as well as the first embodiment.

To etch oxide films in the etcher of the second embodiment, we used agas containing CHF₃ +O₂, CF₄ +CH₂ F₂ or other Freon gas as a base andset the pressure at 0.1 to 10 Pa. The etching rate in these oases was150 to 500 nm/min.

As described above, in this embodiment having the three pairs ofparallel plate electrodes 42 to 47, the high-frequency power with afirst frequencyF is applied on the first pair of parallel plateelectrodes 44 and 46, the high-frequency power with a second frequency2F is applied on the second pair of parallel plate electrodes 45 and 47and a high-frequency power with a different frequency is applied on thethird pair of parallel plate electrodes 42 and 43. According to theembodiment, a plasma with good uniformity is obtained and uniformetching is achieved. Further, since there are almost no localdeviations, damage to devices such as gate oxidefilm breakdown isextremely small. In this embodiment, the ground electrodes (46, 47, 43)have the same potential, but the potential of the ground electrodes (46,47) of the first and second high-frequency power supplies can bedifferent from the potential of the ground electrode (43) of the thirdhigh-frequency power supply.

FIG. 12 shows a plasma etcher of a third embodiment according to theinvention. The plasma etcher comprises: an etching chamber (plasmachamber) 41; a first pair of parallel plate electrodes 44 and 46; and asecond pair of parallel plate electrodes 45 and 47. The two pairs ofparallel plate electrodes 44, 45, 46 and 47 are arranged in the chamber41so that the second pair of parallel plate electrodes 45 and 47 aresubstantially perpendicular with the first pair of parallel plateelectrodes 44 and 46, as shown in FIG. 12. On each of the two pairs oftheparallel plate electrodes 44, 45, 46 and 47, a 300 MHz high-frequencypoweris applied.

The plasma etcher further comprises a sample stage 42 serving as anelectrode on which a 300 MHz high-frequency power is applied, and anopposing electrode 43 which is grounded. The sample stage 42 supports asample (a semiconductor wafer) 3 to be etched. In the space surroundedby the sample stage 42 and the parallel plate electrodes 43 to 47, theplasmais generated. The etching gas is introduced into the chamber 41through a mass flow controller (not shown) and an inlet (not shown). Theinternal pressure of the chamber is maintained at 0.1 to 10 Pa duringetching by a turbo pump (not shown).

Amplifiers 51, 52 and 53 supply high-frequency power to the sample stage42, the parallel plate electrodes 44 to 47 via matching circuits 61, 62and 63. The amplifiers 51, 52 and 53 are controlled by a phase-lockingmechanism 60 so that there is a fixed phase difference (120 degrees)between the high-frequency powers. Thus, the phases of the powersapplied on the parallel plate electrodes 44 and 45 are advanced anddelayed, respectively, 120 degrees with respect to that of the powerapplied on thesample stage 42.

In order to make the frequencies of the high-frequency power equal, asignal generated by a single signal source is used. The signal isamplified to the high-frequency powers (AC powers) having the samefrequency and different phases by the amplifiers 51, 52 and 53.

This embodiment differs from the first embodiment in that 300 MHzhigh-frequency powers whose phases are shifted 120 degrees from eachotherare applied on the parallel plate electrodes 42, 44 and 45. In thisembodiment, the electrons in the chamber are caused to move on asphericalplane by the high-frequency powers. Therefore, a highionization efficiencyand a high plasma density are obtained in spite ofa low pressure.

To etch aluminum in the plasma etcher, a gas containing BCl₃ +Cl₂, SiCl₄+Cl₂ +CHCl₃ or other chlorine gas as a base was used (the pressure wasset at 0.1 to 20 Pa). Under these conditions, an etching rate of 400 to900 nm/min was obtained. As described above, by means of thisembodiment, a mechanism is employed thatimpresses high-frequency powersof the same frequency and whose phases are shifted 120 degrees form eachother on three pairs of parallel plate electrodes, whereby a highlyuniform plasma and uniform etching can be obtained. Further, since thereare almost no local deviations, damage to devices such as gate oxidefilm breakdown in MOS devices is extremely small.

In this embodiment, the sample (wafer) 3 to be etched was placed only onthe parallel plate electrode 42. Since the parallel plate electrodes 44and 45 are equivalent to the parallel plate electrode 42, samples can beetched simultaneously at the parallel plate electrodes 44 and 45,thereby making it possible to increase throughput for etching.

FIG. 13 shows a plasma etcher for dry etching of a fourth embodimentaccording to the invention. The plasma etcher comprises: an etchingchamber (plasma chamber) 71; a pair of parallel plate electrodes 74 and75; a temperature-controlled sample stage 72 serving as an parallelplate electrode on which a 50 MHz high-frequency power is applied; and aground electrode 73 used as its opposing parallel plate electrode. Onthe two parallel plate electrode 74 and 75, a 300 MHz high-frequencypower is applied. The sample stage 72 supports a sample (a semiconductorwafer to be etched) 3. The sample stage 72 is made the coldest part inthe etching chamber and is maintained at 10 to -10 degrees. An amplifier76 supplies ahigh-frequency power to the pair of parallel plateelectrodes 74 and 75 viaa matching circuit 77.

The etching gas is introduced into the chamber 71 through a mass flowcontroller (not shown) and an inlet (not shown). The internal pressureof the chamber is maintained at 0.1 to 30 Pa during etching by a turbopump (not shown). In the space surrounded by the sample stage 72 and theparallel plate electrodes 73 to 75, the plasma is generated.

This embodiment differs from the first embodiment in that there is onlyonepair of parallel plate electrodes on which a 300 MHz high-frequencypower is applied. Further, the parallel plate electrodes 74 and 75 aredisposed outside the etching chamber via a quartz or ceramic material,whereby corrosion due to etching is avoided and metal contamination fromthe parallel plate electrodes 74 and 75 is prevented.

As described in conjunction with the first embodiment, the electrons areoscillated by a high-frequency power applied on the parallel plateelectrodes 74 and 75, and a high ionization efficiency and high plasmadensity are obtained in spite of a low pressure. In order to increasethe ionization efficiency, it is desirable to use a frequency higherthan 50 MHz, but the effectiveness of the invention can still beconfirmed at a lower frequency.

To etch aluminum in the plasma etcher, a gas containing BCl₃ +Cl₂, SiCl₄+Cl₂ +CHCl₃ or other chlorine gas as a base is used. The electric fieldbetween the parallel plate electrodes wasuniform. Therefore, a plasmawith good uniformity was obtained which yielded highly uniform etching.By making the sample stage 72 in this embodiment the coldest part in theetching chamber 71, the selectivity ratio to the photoresist (not shown)on the sample 3 is increased to above5. Further, there are almost nolocal deviations in the plasma, so that damage to the sample 3 is veryslight.

Application of the 50 MHz high-frequency power applied on the samplestage 72 is made so as to form a bias for changing the energy of theions reaching the sample surface in order to control the selectivityratio. Forthis reason, it is generally smaller than the high-frequencypower applied on the parallel plate electrodes 74 and 75 by several tensof watts. Therefore, the amount of current flowing to the sample is onlythe amount required for etching and it can be smaller than the levelrequired to maintain the plasma. By this means, damage due to charge-upis small when compared to cases in which the sample stage is used as aparallel plate electrode for supplying the high-frequency power requiredto maintain the plasma.

As described above, in this embodiment, a mechanism is employed in whicha high-frequency power with a frequency greater than 50 MHz is appliedon a pair of parallel plate electrodes 74 and 75 and a high-frequencypower with a second frequency is applied on another pair of parallelplate electrodes 72 and 73. Thus, a high-density plasma with gooduniformity canbe obtained. Further, since there are almost no localdeviations in the plasma, damage to the processed material is veryslight.

The invention utilizes the phenomenon of the oscillation, revolution orcycloidal motion of electrons in a suitable high-frequency electricfield and applies the high-density plasma, which is highly uniform overa large area, generated at a low pressure for etching. According to theinvention,etching can be realized to form a superior fine patterning.Thus, the plasma etcher of the invention is highly suited to massproduction. According to the invention, an excellent uniformity isattained, and it allows very little damage to devices such as gate oxidefilm breakdown, thus contributing greatly to the production ofhigh-density semiconductor devices.

While the invention has been described in conjunction with the plasmaetcher, the invention is applicable to a plasma CVD reactor, a plasmareactor for surface treatments and so on.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom thescope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. An apparatus for generating plasma comprising:a plasma chamber; a pair of electrodes for generating plasma in said plasma chamber; first power supply means for applying a first high-frequency power to said pair of electrodes, to produce an oscillation electric field in a space formed by said pair of electrodes; a holder for holding at least one wafer, said holder disposed in said plasma chamber and provided outside said space formed by said electrodes, said holder having a holding face which faces said space, said holding face being substantially parallel to the direction of said oscillation electric field; and second power supply means for applying a second high-frequency power to said holder, to supply said holder with a bias voltage, wherein the period of said first high-frequency power is shorter than the period of said second high-frequency power, and is shorter than the time required for electrons contained in said plasma to travel between said pair of electrodes.
 2. An apparatus according to claim 1, wherein a distance between said electrodes is longer than a length between both ends of said holding face of said holder.
 3. An apparatus according to claim 1, wherein said first high-frequency power has a frequency greater than 50 MHz.
 4. An apparatus according to claim 1, wherein said pair of electrodes is provided inside said plasma chamber.
 5. An apparatus according to claim 1, wherein said pair of electrodes is provided outside said plasma chamber.
 6. An apparatus according to claim 1, further comprising temperature control means for regulating the temperature of said holder.
 7. A plasma etcher comprising:a plasma chamber a pair of electrodes for generating plasma in said plasma chamber; first power supply means for applying a first high-frequency power to said pair of electrodes, to produce an oscillation electric field in a space formed by said pair of electrodes; a holder for holding at least one wafer, said holder disposed in said plasma chamber and provided outside said space formed by said electrodes, said holder having a holding face which faces said space, said holding face being substantially parallel to the direction of said oscillation electric field; and second power supply means for applying a second high-frequency power to said holder, to supply said holder with bias voltage, wherein the period of said first high-frequency power is shorter than the period of said second high-frequency power, and is shorter than the time required for electrons contained in said plasma to travel between said pair of electrodes.
 8. A plasma etcher according to claim 7, wherein a distance between said electrodes is longer than a length between both ends of said holding face of said holder.
 9. A plasma etcher according to claim 7, wherein said first high-frequency power has a frequency greater than 50 MHz.
 10. A plasma etcher according to claim 7, wherein said pair of electrodes is provided outside said plasma chamber.
 11. A plasma etcher according to claim 7, further comprising temperature control means for regulating the temperature of said holder. 